Detector distortion control



Sept. 8, 1936.. I Q v s 2,053,449

DETECTOR DISTORT ION CONTROL Filed Feb. 28, 1955 CHARLES TRAVIS BY 7 4M ATTORNEY.

Patented Sept. 8, 1936 UNITED STATES DETECTOR DISTORTION CONTROL Charles Travis, Philadelphia, Pa., aassignor to Radio Corporation of America, a corporation of Delaware Application February 28, 1935, Serial No. 8,614

'6 Claims. (OI. 250-'-27) My present invention relates to arrangements for minimizing distortion in radio receivers,and more particularly to a novel and improved type of detector circuit adapted for use in connection with radio broadcastreceivers.

The present tendency in the art of radio broadcasting is to approach as closely as possibleto modulation of the. radiated carrier wave. The audio waves picked up bythe broadcasting station microphone are thus faithfully radiated by means of the radio frequency carrier wave. In order to warrant such high fidelity transmission, it is obviously necessary to utilize at the receiving point a receiving arrangement which is capable of" high fidelity reproduction. That is to say, the receiver should, for so-called 'high fidelity reception, be capable of faithfully reproducing the audioenvelope of the collected modulated carrier waves.

One of the most importantflinks in the chain of networks comprising a high fidelity radio re-' 'ceiver is the detector network. It is essential in a receiver of this type that the detector network possess a substantially linear detection characteristic up to 100% modulation of the received carrier wave. The only demodulation device known in the past which approximates this ideal operation is, the diode detector. However, a diode detector possesses various inherent disadvantages which render it undesirable for use in high fidelity reception. j

Accordingly, it may be stated that'it' is one of the main objects of the present invention to providea detection network which isreadily varied in its detection characteristic so.that the characteristic may be substantially linear up to 100%. modulation of the received carrier wave, or alternatively may have itssensitivity greatly increased, and still be substantially linear up' to approximately 50 to '70 modulation of the carrier, the change in detection characteristic being accomplished in a simple and effective manner and being dependent upon whether or not carrier waves are being received from a weak or a strong station.

Another important object of the invention is to providein a degenerative plate circuit detector network a device for varyingthe effective detection characteristic. of the detector network in such a manner that for local broadcast reception of carrier waves of strong amplitude thedetection characteristic issubstantially linear up-to quencies whereby the input electrodes of the detector network have an audio voltage impressed upon them in degenerative phase with respect to the useful audio output voltage wherebythe detection characteristic is substantially linear up to 100% modulation of the received carrier wave; the input circuit of the detector network, additionally, including a controldevice which functions to vary the degree of audio degeneration introduced into the input circuit of the detector whereby the linearity of the detection characteristic with respect to percentage modulation of the carrier wave is capable of being varied, the signal frequency network feeding the detector input circuit having associated therewith an automatic gain control circuit for maintaining the carrier amplitude at the detector input circuit substantially constant over a wide range of signal carrier amplitude variation at any predetermined setting of the audio degeneration control device.

prove generally the simplicity and emciency of detector control devices for radio receivers, and more especially to provide a manually adjustable sensitivity-distortion control device for a high fidelity broadcast receiver which is not only reliable and efiicient in operation, but can be readily manufactured and assembled in a radio receiver.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the accompanying drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Referring now to the accompanying drawing, there is shown in schematic fashion the intermediate frequency amplifier and second detector networks of a superheterodyne receiver which embodies the present invention. It is to be understood that the input of the intermediate frequency amplifier l is connected tothe usually preceding networks of such a receiver, and such networks ordinarily comprise a signal collector, such as a grounded antenna circuit, and a first detector network. The latter maybe of the composite local oscillatorfirst detector type, or ofthe type which uses independent local oscillator and first detector tubes. The network I may'comprise one, or more,,stages-of intermediate frequency amplification, and each stage may utilize a screen grid, or pentode,tube I and these tubes may also be of the variable mu type, if desired.- The second detector network is shown as comprising a tube of the 51, or pentode, type, and the resonant input, circuit 2 of the detectortube 3 is fixedly-tuned to. the operating intermediate frequency. The input circuit 2 is coupled, as at M1, to the resonant output circuit 4 of the intermediate frequency amplifier network I, and the circuit 4 is also tuned to the operating intermediate frequency.

The coupling M1 is optimum so that the overall resonance curve characteristic of the coupling network is substantially band pass in nature, this lending itself to high fidelity reception. The detected output of the detector tube 3 is transmitted to the succeeding audio frequency transmission network through condenser 5, and it is to be understood that the audio network may comprise one, or more, stages of audio frequency amplification, and the final stage of the audio network may be coupled to anydesired type of reproducer. It is to be understood that the networks, other than the detector, are

designed for high fidelity reception. Such design usually includes a control for regulating the selectivity characteristic of a signal selector network. For example, those skilled in the art are aware that the I. F. network preceding the detector may have its selectivity adjustable so that when weak stations are received the said net work will be sharply selective; on the other hand when strong local stations are received the selectivity will be varied to impart a wide band pass characteristic to the network. In receivers of this type it is important to have the demodulator characteristic varied at the same time, since for weak station reception it is more important to have maximum gain at the detector than minimum harmonic distortion.

Considering, now, the specific construction of the second detector network, it will be noted that the cathode circuit of the detector tube 3 includes a grid bias resistor 6 having one side of the detector input circuit 2. The grounded screen grid lead of detector tube 3 includes the positive potential source S, and the anode of the detector tube is connected to ground through a path which includes the detector plate circuit resistor 8 and the positive plate potential source 13.

There is connected in shunt with the grid bias resistor 6 a radio frequency by-pass condenser 9, and the anode circuit lead of the detector tube is by-passed to ground for radio frequencies through the condenser II). A condenser II is connected between the low alternating potential side of the detector input circuit 2 and an adjustable tap I2, the tap being slidable along the grid bias resistor 6. The condenser II is an audio frequency by-pass condenser, and may have a magnitude of about 0.01 mfd. When the tap I2 is at the cathode side of the grid bias resistor 6, it will be clear that the resistor 6 is by passed for audio frequencies. However, when the tap I2 is moved towards the grounded side of resistor 6 an audio voltage is developed across the resistor 6, and this audio voltage is impressed upon the input electrodes of the detector tube in degenerative phase to the audio voltage developed in the output circuit of the detector.

In order to maintain the carrier amplitude at the detector input circuit 2 substantially uniform over a wide range of carrier amplitude variation, the automatic volume control system I3 is provided. This automatic volume control system is shown connected in such a manner that it derives its signal energy from the primary circuit of the coupling M1, and its variable bias output is applied to any of the stages preceding the second detector. The signal input for the automatic volume control tube is not derived from the second detector network in order that the latter shall not be loaded, and also in order to have the selectivity at the detector input ofa higher degree than that at the automatic volume control input. Those skilled in the art are fully aware of the advantage of such an arrangement, and it need not be discussed in any further detail.

The specific construction of the automatic volume control arrangement I3 need not be described because any system well known to those skilled in the art may be utilized for this purpose. How-ever, in order to render the present disclosure complete, reference is made to the fact that there may be utilized for the automatic volume control system I3 the arrangement disclosed by Stuart Ballantine in application Serial No. 376,163, filed July 5, 1929. In this aforesaid application there is disclosed an automatic volume control arrangement which is highly suitable for the present purposes, and which will maintain the carrier amplitude at the detector input circuit'substantially uniform over a wide range of received carrier amplitude variation.

When the tap I2 is adjusted to'the grounded side of the grid bias resistor 6, the latter is substantially unby-passed for audio frequencies. In this position of the tap I2 the detection characteristic is substantially linear up to modulation of the received carrier waves. In such position of tap I2 the resistor 6 may have a magnitude of the order of 100,000 ohms, and. the condenser 9 is to be understood as furnishing solely radio frequency b y-passing, this by-passing action being negligibly small for audio frequencies. The resistor I may have a magnitude of the order of 1 megohm, and the plate circuit resistor 8 is given a magnitude which is larger than the cathode resistor 6; the latter may be done if the screen electrode of tube 3 is by-passed to ground for radio frequencies. The detector tube 3 then acts as a triode (cathode, control grid, screen) with the plate electronically coupled. This detector circuit reverts to the' ordinary type of screen grid detector when the tap I2 is adjusted to the cathode side of resistor 6 for maximum detector gain. When tap I2 is adjusted to the last position the significance of by-passing the screen grid to ground disappears.

When the tap I2 has been adjusted to give degenerative action, the detector circuit has been adjusted for minimum harmonic distortion. In view of the fact that with this setting of the tap I2 the detection. characteristic is substantially linear up to 100% modulation of the received carrier, it will be seen that the operation is similar to that of a diode rectifier circuit without the disadvantages inherent in the latter. As a matter of fact, in normal operation, the degenerative plate circuit detector acts like a peak rectifier in the manner of a diode, but unlike the diode it draws no input power. Its dynamic characteristic properties are identical with that of a diode detector circuit, with the exception that in a practical diode circuit the driver impedance is of the same order as the input impedance of the diode circuit.

In the case of the degenerative plate circuit detector increased gain and selectivity is secured due to the fact that a negligibly high input impedance is utilized. It can be readily demonstrated that for high quality detection, when the tap I2 is adjusted to the grounded side of grid bias resistor 6, the degenerative plate circuit detector more closely achieves ideal detection operation than a diode detector circuit. Furthermore, by virtue of the electronically coupled output, there is secured a higher gain while retaining the distortion-free detection characteristic.

The degenerative action of the plate circuit detector shown can be reduced, or removed, if it is desired to increase detection gain at the expense of harmonic distortion. By moving the tap l2 towards the cathode side of the grid bias resistor 6 the degree of audio degeneration introduced into the detector input circuit is reduced. At the cathode terminal of resistor 6 the degenerative action is entirely eliminated, and the audio frequencies are entirely by-passed through the condenser II. In this position of the tap l2 the detector circuit functions in the normal and well known plate rectification manner and gives high gain, as well as substantially linear detection up to 50 to 70% modulation of the received carrier wave. For distant reception, or local reception of carriers of weak amplitude, a detection characteristic which is linear up to 50 to 70% modulation is sufiicient since high sensitivity is desired in such cases. Hence, it will be seen that the control of audio degeneration introduced into the detector input circuit is an extremely useful device to employ in a high fidelity receiver, since in the reception of weak stations the high quality detection function can be reduced to a considerable extent in order to achieve greater gain at the demodulator.

The automatic volume control system l3 performs a highly desirable function in conjunction with the manually adjustable audio degeneration detector control device, aside from its well known function of counteracting fading effects. It is desired, at any setting of the adjustable tap 12, that the carrier amplitude at the detector input circuit be substantially constant. TlL's will now be explained. The operating bias on the signal input grid of detector tube 3 is composed of two components; one of these is the direct current component developed by the carrier current flow through the bias resistor 6, and the other is the audio voltage component developed by the audio frequency currents flowing through the resistor 6. For any given setting of the tap 12 on resistor 6, there is a critical relation between the magnitude of that portion of the resistor 6 between tap l2 and the cathode terminal of resistor 6 and the carrier amplitude at the detector input circuit.

As soon as the carrier amplitude varies in value at the detector input circuit, this aforesaid relationship is upset, and it would be necessary to adjust the tap l2 to another point on the bias resistor 6. However, by utilizing the automatic volume control arrangement, IS, the carrier amplitude value at the detector input circuit is kept substantially constant, and the taplZ may be adjusted to a desired point on resistor 6 for a desired quality of detection, and it will not be necessary to shift the tap again because of carrier amplitude variation. Furthermore, it will be noted that the operation of the automatic volume control system l3 in no way affects the functioning of the degenerative plate circuit detector. While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

I claim:

1. In a radio receiver including a signal amplifier and a detector of the plate rectification type, a grid bias resistor in the cathode circuit of the detector tube, said resistor being common to the detector input and output circuits, means for varying the audio frequency by-passing of'the said grid bias resistor whereby the degree of audio degeneration of the detector input circuit may be varied, and additional means for maintaining the received carrier amplitude at the detector input circuit of a substantially constant value for any desired setting of said audio frequency bypassing means.

2. In a radio receiver of the high fidelity reception type, a detector including a pentode tube, a grounded grid bias resistor in the cathode circuit of said tube, a signal input circuit connected between the signal input grid and the grounded side of said grid bias resistor, said resistor being common to the detector input and output circuits, an audio frequency by-pass condenser having one side thereof fixedly connected to the detector input circuit and the other side thereof connected to the said grid bias resistor through an adjustable tap whereby the degree of audio degeneration introduced into the detector input circuit may be varied at will.

3. In a receiver as defined in claim 2, means for by-passing the screen grid electrode of the detector tube to ground for radio frequencies.

4. In a radio receiver, a detector including a pentode tube, a grounded grid bias resistor in the cathode circuit of said tube, a signal input circuit connected between the signal input grid and the grounded side of said grid bias resistor, said resistor being common to the detector input and output circuits, and an audio frequency by-pass condenser having one side thereof fixedly connected to the detector input circuit and the other side thereof connected to the said grid bias resistor through an adjustable tap whereby the degree of audio degeneration introduced into the detector input circuit may be varied at will, and means for maintaining the received carrier amplitude at the detector input circuit substantially uniform over a wide range of received carrier amplitude variation.

5. In a receiver as defined in claim 4, the screen grid electrode of the detector tube being by-passed to ground for radio frequencies.

6. In a detector circuit of the type including a tube, a signal input circuit and an audio output circuit, a grid bias resistor in the cathode circuit of the tube, said resistor being common to the detector input and output circuits, a resistive path connected between the low alternating potential side of the detector input circuit and the negative side of said grid bias resistor, and means for controlling the linearity of the detection characteristic with respect to percentage modulation of the received carrier wave, said means comprising an audio frequency by-pass condenser having one terminal thereof connected to the detector input circuit and the other terminal thereof connected to the grid bias resistor through an adjustable tap.

CHARLES TRAVIS. 

